The present invention relates to a halftone image printing device, and, more particularly, to a halftone image printing device in which a pixel is formed by a plurality of dots in a matrix form, and can be printed in a halftone mode, or with gradation or gray levels.
In a halftone image printing device, a thermal head (printing head) is urged against printing paper through an ink film (normally having a ribbon-shape), and an ink on the ink film is melted by heat generated when heating resistors constituting the thermal head are energized. Thus, the melted ink is transferred to the printing paper so as to form a dot image corresponding to the energized resistors. In this apparatus, each dot can only be binary-controlled as to whether or not the ink is transferred. Therefore, in order to print a halftone image such as a picture, a so-called binary area modulation method is generally adopted. In this method, a pixel must correspond to a plurality of dots in a matrix form. The number of dots which are energized and subjected to ink transferring, however, changes in accordance with the density of a pixel. A DITHER method, a micro-font method or the like are well known as binary area modulation methods.
However, the number of levels able to be represented by this area modulation method is limited. When a pixel has an n.times.n dot matrix configuration, the number of levels expressed is n.sup.2 +1, including 0 level (the level of the printing paper). For example, in the case of a 4.times.4 dot matrix, 17 levels are provided. In general, a color image requires a resolution of 4 dots/mm or higher, and each color component requires 64 gray levels or more. In order to satisfy these requirements with the above-mentioned area modulation method, a pixel must be configurated by an 8.times.8 dot matrix, and a thermal head having a resolution of 32 dots/mm or higher is needed. Although a thermal head having a resolution of 16 dots/mm has been developed, it is difficult to realize one having a resolution of 32 dots/mm or higher. For this reason, in this area modulation method, requirements for the number of gray levels and resolution cannot be satisfied, and it is impossible to perform halftone printing having a gradation that in both smooth and fine.
The above description is made on monochromatic image printing. However, a full-color image can be printed by superposing images of a plurality of (generally three or four colors) color components. More specifically, inks of a plurality of colors are transferred in a superposed manner in amounts corresponding to the densities of the respective color components for each pixel. Then, dots of the respective colors in densities corresponding to the densities of the respective colors are superposed and formed as a single pixel on a printing sheet. In this case, when the transfer positions of the dots of the respective colors are misregistered, the hue is greatly changed. Therefore, in order to obtain a printed image with a stable hue, the transfer positions of the respective color inks must be correctly controlled.
However, it is considerably difficult to correctly position the transfer positions of the respective color inks when mechanical errors of the printer and high resolution are considered. As a result, conventionally, the reproduced hue is not stable because of the adverse influence of variations in relative position of the printer head and the printing sheet.
Such a mechanical error of the printer causes a slight skew, that is, rotation of the texture direction (the direction of a continuous line which the printed dots appear to form) of the printing dots of the respective color inks, thereby causing color moire by the printed dot patterns of the respective color inks. More specifically, since dots are printed in a lattice-like manner, when the lattice directions of the respective colors are misregistered, dot arrays intersect at positions different from the original positions. These intersections are cyclically aligned, and moire fringes are formed.